Shear velocity tomography of the crust below Valles Caldera using a densely spaced nodal transect

Abstract:

Valles Caldera is a volcanic caldera located at the intersection of the Jemez lineament intraplate volcanic fields and the Rio Grande rift in north-central New Mexico. The most recent caldera forming eruption, ~ 1.25 Ma, had a volcanic explosivity index ≥ 7 and erupted ~350 km³ dense rock equivalent with primarily rhyolitic composition. The youngest rhyolite flows in the caldera are ~70 ka, suggesting the potential for magmatic recharge since the last caldera forming eruption. Understanding the current state of such large explosive volcanic systems is societally important given the potential for devastating environmental effects at local to regional scales and more moderate hazards extending to greater distances. The size of the magmatic systems necessary for large caldera forming eruptions is conducive to seismic imaging techniques, especially if dense array data exist. A previous P-wave tomography study identified a low velocity zone in the middle crust below Valles Caldera. However, no models of shear wave velocity, which is more sensitive to the presence of melts than P-wave velocity, exist for Valles Caldera. In the fall of 2019 we deployed 97 three-component nodal seismometers with an average spacing of ~750 m along a ~71 km SSW-NNE striking linear transect over Valles Caldera. The work presented here aims to use this dataset to create a high resolution cross-sectional (2D) shear wave velocity model of the crust below Valles Caldera. Cross correlations of continuous ambient noise data recorded on the densely spaced stations show clear Rayleigh (ZZ) and Love (TT) wave arrivals. These measurements will be combined with correlations between the nodal array and regional broadband seismographs to increase the bandwidth of dispersion curves for shear velocity tomography inversions. Beamforming with dense subsets of the nodal array will be used to constrain Rayleigh and Love wave phase velocities. Shear wave velocity will be calculated with (an)isotropic Bayesian Markov chain Monte Carlo inversions. The new images should improve constraints on the presence, depth interval, and melt content of the contemporary magmatic system beneath Valles Caldera.